It is known that there are essentially two transmission modes in telecommunications networks:                a transmission mode referred to as “circuit mode”, in which transmission resources are allocated for the duration of a call, regardless of whether or not any information is transmitted; and        a transmission mode referred to as “packet mode”, in which transmission resource allocation can be modified during a call, thereby enabling the transmission resources to be shared better between a plurality of calls, as a function of the instantaneous needs of each of them.        
Circuit mode is more particularly suited to voice calls, whereas packet mode is more particularly suited to data transmission, in particular when transmission rates are not uniform over time, and also, when they are asymmetrical in the two transmission directions, which is the case in particular when the network is used to transmit data between a telecommunications terminal and a data-processing server.
The present invention relates more particularly to the use of packet mode in mobile telecommunications networks, such use also being known under the name “General Packet Radio Service” (GPRS), for mobile telecommunications networks of the “Global System for Mobile communications” (GSM) type.
It is recalled that the GSM network is a network of the “Time Division Multiple Access” (TDMA) type, in which transmission resources are constituted, as shown in FIG. 1, by time channels or time slots (IT), referenced in FIG. 1 merely by their numbers 0 to 7, in down frames (such as TD0) used for transmission in the network-to-mobile station direction, and in up frames (such as TM0) used for transmission in the mobile station-to-network direction, the down frames and the up frames being carried by distinct carrier frequencies, respectively a “down” frequency and an “up” frequency.
In a GSM network, the same time slot is allocated for a circuit-mode call both in the down frames and in the up frames, where the up frames are also offset in time relative to the down frames, as shown in FIG. 1, by a duration enabling a mobile station operating in “half-duplex” mode to perform the necessary frequency switching, such a mobile station operating in half-duplex mode being equipped with means making it possible to receive on the down frequency and to transmit on the up frequency alternately and not simultaneously.
When the GPRS is used, the time slots allocated to a mobile station for the duration of a call can change from one frame to another. Furthermore, it is known that it is possible to inform the mobile station of the time slots that are allocated to it in the up direction, for a given frame, by addressing a transmission authorization in the down direction, during the preceding frame, and on each of the same time slots, the transmission authorization in practice being contained in a particular bit field referred to as the “Uplink Status Flag (USF)”.
With such an authorization mechanism, it can be seen that, to increase the number of consecutive time slots allocated for transmission, it is necessary to increase correspondingly the number of consecutive time slots allocated for reception, but that such an increase is limited by half-duplex mode operation which makes it necessary to provide a certain amount of guard time between the time slots allocated for reception and those allocated for transmission, so that the necessary frequency switching can be performed.
For example, to allocate the four consecutive time slots IT0 to IT3 to a mobile station for transmission, it would be necessary to allocate the four consecutive time slots IT0 to IT3 to it for reception, which, in practice, is impossible for a standard-type mobile station (i.e. a mobile station equipped with a single frequency synthesizer, i.e. requiring a guard time equal to two time slots), as can be seen from FIG. 1.
Thus, in the context of the GSM network and of standard-type mobile stations operating in half-duplex mode, the maximum number of consecutive time slots allocated per frame for reception is equal to four, and the maximum number of consecutive time slots allocated per frame for transmission is equal to three (four consecutive time slots allocated for reception corresponding to two consecutive time slots allocated for transmission, and three consecutive time slots allocated for reception corresponding to three consecutive time slots allocated for transmission).
To avoid that drawback, and therefore to increase the transmission capacity in each frame, it has been proposed to give transmission authorizations once per group of consecutive frames rather than once per frame (i.e. rather than “in-frame”), the authorizations then being given in particular signalling messages referred to as “Fixed Assignment” messages. With such a method, it is then possible to allocate whole frames for transmission (or for reception).
Unfortunately, such a solution also suffers from drawbacks.
In particular, the allocation scheme chosen for an allocation period corresponding to a group of consecutive frames can no longer be modified for the entire period. Unfortunately, allocation needs can change, in particular to satisfy more urgent requests occurring during said period, in which case the initial allocation scheme can be inappropriate. In other words, lengthening the allocation period gives rise to a lack of reactivity from the network.
Furthermore, if the mobile station remains in transmission for too long, in a cellular network such as the GSM network in particular, it can longer listen regularly to neighboring cells, in order to perform measurements firstly to determine which of the cells is the best and is therefore the cell to which the call could be handed over during a handover procedure, and secondly to locate the mobile station, and thus to make it possible to control its transmission power in order to reduce the overall level of interference in the network.
Naturally, provision can be made to reserve certain groups of consecutive frames for performing such measurements on the neighboring cells, but that leads to such measurements being concentrated in time, which means that the results obtained are not properly representative (ideally, to be genuinely representative, the measurements should be distributed over all of the frames).
In addition, to enable the network to determine the number of frames to be reserved for such measurements, the mobile station must transmit to the network the specific signalling messages indicating in particular the number of neighboring cells, the speed at which the mobile station can perform the measurements, etc, this being to the detriment of the payload data to be transmitted.